Thermoplastic powders or slurries, known as dry or liquid plastisols, are conventionally cast on the inner surface of a thin metal mold member heated to the fusion temperature of the deposited material to form a gel coat which is heated sufficiently to form a thin, solid thermoplastic molded member (i.e. a shell or skin). The thin metal mold member remains heated until a thin layer of thermoplastic material is built up on the inner surface to form a thin plastic shell product, such as a product suitable for use as an outer covering on automotive interior products such as door panels, instrument panels, or the like.
The method and apparatus for the production of molded skins or shells are well known, such methods often being referred to as "rotational" or "slush" molding. The mold cavity defined by the mold shell is filled with a flowable plastic material and, due to heating of the mold shell, a thin layer of plastic material adheres to the surface of the mold shell and the remaining plastic material can then be shaken or poured out of the mold, as by rotating the mold. When the mold shell is further heated, the skin or shell product clinging thereto gels out. The finished skin or shell product can then be removed after the mold shell cools down. The operation of introducing the plastic material, when in the form of a powder or granule, is typically achieved by rotating the mold shell so that the mold cavity thereof opens downwardly, and then fixing onto the mold shell a box which contains the plastic material. The overall assembly consisting of the mold shell and mold box is then rotated through about 180.degree. so that the plastic material drops into the mold shell. After the plastic skin or shell has set against the heated mold surface, the mold assembly is rotated back to its original position and the mold box and excess powder removed, with further heating of the mold shell then effecting proper melting and gelling of all of the plastic material adhered to the mold surface. The mold shell is then cooled and the molded skin or shell product removed.
In a typical rotational molding apparatus of the type described above, the mold assembly includes a generally hollow mold housing or box on which the metal mold shell is mounted, which housing and mold shell cooperate to define a hollow interior chamber to which heated air is supplied so as to effect heating of the mold shell from the back surface thereof. This heating of the mold shell, however, has long presented a problem since the irregularity in the shape of the mold shell and the length thereof have often made it difficult to provide the desired degree of heat uniformity as applied to the mold shell throughout the back surface area thereof. The mold shell thus often has undesired hot or cold spots which effect the quality (i.e. thickness and/or finished surface properties) of the molded skin product.
In an attempt to provide better control over the heat applied to the back surface of the mold shell, various flow arrangements have been proposed, including blowing heated air into one end of the mold housing and controlling the flow of the air therethrough with various air flow control devices. Cooler air then exits the opposite end of the mold housing. However, in this arrangement temperature and velocity drops occur from one end of the mold housing to the other as the air moves therethrough, and this can result in unsatisfactory and uneven heating of the mold shell.
It is an object of the invention to provide an improved molding apparatus and method, such as a slush or rotational molding apparatus and method, for forming a thin skin or shell product, which improved method and apparatus is believed to significantly improve the overall performance characteristics of the apparatus and process in terms of efficiency and rate of productivity. The invention provides more efficient heating of the back surface of the mold shell by utilizing an air heating and supply arrangement capable of providing heated air in a more uniform manner along substantially the entire longitudinal extent of the mold housing or box, and a flow control arrangement for directing the heated air towards the back of the mold shell, which flow control arrangement is capable of producing greater air velocity to provide increased air flow and more efficient heating of the mold shell while requiring less heat.
In accordance with the present invention, the improved mold arrangement includes a mold housing defining therein a generally hollow interior and having a mold shell mounted thereon which defines a mold cavity which opens outwardly and is defined by a front surface of the mold shell. The mold housing has a wall structure disposed therewithin which cooperates with a back surface of the mold shell to define an interior air chamber. The wall structure defines a plurality of discharge and feed channels. The feed channels communicate with a source of heated air and the discharge channels communicate with the air chamber. Flow control devices individually communicate with the discharge channels for directing heated air fed thereto from a corresponding one of the feed channels into the air chamber and toward the back surface of the mold shell.
Another aspect of the invention resides in a molding arrangement which includes a mold housing defining therein a generally hollow interior and having a mold shell mounted thereon defining a mold cavity which opens outwardly and is defined by a front surface of the mold shell. The mold housing and a back surface of the mold shell together define an interior air chamber, and the mold housing defines therein a plurality of discharge and feed channels, the discharge channels opening into the air chamber. The mold arrangement includes an elongate tubular member having a portion which extends within the interior of the mold housing and along substantially the entire length thereof. A plurality of openings are provided within this portion of the tubular member and are each aligned with a respective one of the feed channels. The molding arrangement also includes a plurality of air control devices each disposed to communicate with one of the discharge channels for directing heated air fed to the flow control device from a corresponding one of the feed channels into the air chamber and toward the back surface of the mold shell. A heat source including an arrangement for generating heated air within the portion of the tubular member located within the interior of the mold housing provides a uniform flow of heated air to the feed channels through the openings of the tubular member.
Yet another aspect of the invention resides in a mold arrangement which includes a mold housing defining therein a generally hollow interior and having a mold shell mounted thereon which defines a mold cavity which opens outwardly and is defined by a front surface of the mold shell. The mold housing has a wall structure disposed therewithin which cooperates with a back surface of the mold shell to define an interior air chamber. The wall structure includes a plurality of partition walls disposed transversely within the mold housing and defines a plurality of discharge and feed channels arranged in an alternating manner along the longitudinal extent of the mold housing. The discharge channels open into the air chamber and the feed channels communicate with a source of heated air. The mold arrangement also includes a plurality of flow control devices, with at least one flow control device being disposed within each of the discharge channels for directing heated air fed to the flow control device from a corresponding one of the feed channels into the air chamber and toward the back surface of the mold shell. Each flow control device is supported for rotation about an axis substantially parallel to the lengthwise extent of the mold shell.
A further aspect of the invention resides in a method for molding a thin thermoplastic shell including the steps of: providing an elongate tubular member disposed within the interior of the mold housing and extending along substantially the entire longitudinal extent thereof; providing a plurality of centrifugal fans at spaced-apart locations from one another within discharge channels defined within the mold housing which open into the air chamber; supplying heated air into the tubular member and discharging heated air along substantially the entire longitudinal extent thereof directly into feed channels defined within the mold housing between adjacent pairs of discharge channels; and discharging heated air at a high velocity into the air chamber and along the back surface of the mold shell.
The advantageous arrangement and method of the present invention, and the objects and purposes thereof, will be apparent to persons familiar with molding of this general type upon reading the following description and inspecting the accompanying drawings.